The following is the most accepted definition of the poetic form limerick.

Message 1173: Limerick Form
AngelPie_Mouse
July 08, 1999 10:56 pm EDT

LIMERICK

A light or humorous verse form of five chiefly anapestic (see below) verses of which lines one, two and five are of three feet and lines three and four are of two feet, with a rhyme scheme of aabba.

The limerick, named for a town in Ireland of that name, was popularized by Edward Lear in his Book of Nonsense published in 1846.

Sidelight: As shown by these examples, limericks, while unsuitable for serious verse, lend themselves well to clever wit and word-play. Their content also frequently tends toward the ribald and off-color.

Examples:

There was a young lady of Lynn Who was so uncommonly thin That when she essayed To drink lemonade She slipped through the straw and fell in. -- Anonymous

There was a young lady of Niger Who smiled as she rode on a tiger. They returned from the ride With the lady inside And the smile on the face of the tiger. -- Anonymous

There was a young lady named Bright, Who traveled much faster than light. She started one day In the relative way, And returned on the previous night. -- Anonymous

ANAPEST, ANAPESTIC

A metrical foot with two short or unaccented syllables followed by a long or accented syllable, as in intervene or for a while.

FOOT

A unit of rhythm or meter, the division in verse of a group of syllables, one of which is long or accented. For example, the line,

"The boy | stood on | the burn | ing deck,"

has four iambic metrical feet. The fundamental components of the foot are the arsis and the thesis. The most common poetic feet used in English verse are the iamb, anapest, trochee, dactyl and spondee, while in classical verse there are 28 different feet.

The other metrical feet are the amphibrach, antibacchius, antispast, bacchius, choriamb, cretic, diiamb, dispondee, dochmius, molossus, proceleusmatic, pyrrhic and tribrach, plus two variations of the ionic, four variations of the epitrite, and four variations of the paeon. The structure of a poetic foot does not necessarily correspond to word divisions, but is determined in context by the feet which surround it.

Sidelight: A line of verse may or may not be written in identical feet; variations within a line are common. Consequently, the classification of verse as iambic, anapestic, trochaic, etc., is determined by the foot which is dominant in the line.

Sidelight: To help his young son remember them, Coleridge wrote the poem, Metrical Feet.

Information Source: Glossary of Poetic Terms from BOB'S BYWAY [URL: http://shoga.wwa.com/~rgs/glossary.html]